1. Field of Invention
This invention relates generally to communications, and more particularly, to a method and apparatus for providing digital subscriber line (xDSL) communications between analog front ends (AFEs) and one or more digital signal processors (DSPs).
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as xDSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An xDSL modem operates at frequencies higher than the voiceband frequencies, thus an xDSL modem may operate simultaneously with a voiceband modem or a telephone conversation.
Currently there are over ten discrete xDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrierless AM/PM (CAP). A typical DMT system utilizes a transmitter inverse fast Fourier transform (IFFT) and a receiver fast Fourier transform (FFT). The following patents are related to DMT modems: U.S. Pat. No. 5,400,322 relates to bit allocation in the multicarrier channels; U.S. Pat. No. 5,479,447 relates to bandwidth optimization; U.S. Pat. No. 5,317,596 relates to echo cancellation; and U.S. Pat. No. 5,285,474 relates to equalizers. The following patents are related to CAP modems: U.S. Pat. No. 4,944,492 relates to multidimensional passband transmission; U.S. Pat. No. 4,682,358 relates to echo cancellation; and U.S. Pat. No. 5,052,000 relates to equalizers. Each of these patents is incorporated by reference as if fully set forth herein.
XDSL modems are typically installed in pairs, with one of the modems installed in a home and the other in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home from a line card at the central office on which the modem is implemented through the subscriber line or local loop.
Modems using CAP or DMT or other line codes, essentially have three hardware sections:    (a) an analog front end (AFE) to convert the analog signals on the subscriber line into digital signals and convert digital signals for transmission on the subscriber line into analog signals,    (b) digital signal processing (DSP) circuitry to convert the digital signals into an information bitstream and optionally provide error correction, echo cancellation, and line equalization, and    (c) a host interface between the information bitstream and its source/destination. Typically all of these components are located on a highly integrated single line card with a dedicated connection between one or more AFE's and a DSP.
Each installation represents a sizeable expense in hardware and service labor to provision the central office. The expense may not always be amortized over a sufficient period of time due the relentless introduction of new and faster xDSL standards each of which pushes the performance boundaries of the subscriber line in the direction of increasing bandwidth and signal integrity. As each new standard involves, line cards must typically be replaced to upgrade the service.
What is needed is a less rigid signal processing architecture that supports scalability of CO resources, and allows a more flexible hardware response to the evolving xDSL standards and the problems associated with providing hardware to handle each new standard.